1. FIELD OF THE INVENTION
The invention relates to a method for demodulating an FM signal (FM=Frequency Modulation), in which the FM signal is directly mixed down into an in-phase and a quadrature component of a complex frequency baseband signal and a modulation signal is determined from a phase relationship of the complex frequency baseband signal.
2. DISCUSSION OF BACKGROUND
As mobile radio units become increasingly more widely distributed, so does the demand for added features. Such units should not only have good reception characteristics but should also be small, of light weight and compact. It is therefore important that the technology of highly integrated circuits can be advantageously utilized for the demodulation of FM signals. It has now been found that signal receivers operating in accordance with the principle of direct conversion are the most suitable for this so-called integration. That is to say, an FM signal is mixed down from a carrier frequency into the frequency baseband without using any intermediate frequency stages.
However, a frequency baseband signal obtained in this manner must still be amplified in most cases for further processing. To avoid offset problems, AC-coupled amplifiers are usually used.
In the printed document "A radio-paging receiver architecture and Demodulator" by C. B. Marshall, Philips Journal of Research, Vol. 41, Supplement No. 2, 1986, a signal receiver for the demodulation of digital data is disclosed which operates in accordance with the principle of direct conversion. If it is intended to use such a signal receiver for the demodulation of FM signals, signal distortions occur which mainly originate from the AC-coupled amplifier. In contrast to digital signals, analog signals have direct-current components which cannot be neglected. Due to the fact that these are suppressed by an AC-coupled amplifier, considerable signal distortions arise, for example after pauses in speaking.